1. Field of the Disclosure
This disclosure relates generally to drilling or milling devices for drilling boreholes in earth formations using same.
2. Brief Description of the Related Art
Boreholes are drilled in earth formations using a drilling system for a variety of purposes, including production of hydrocarbons, such as oil and gas. A drilling system typically includes a drill string that includes a drilling tubular having a drilling assembly at the end of the tubular. In some cases the drilling assembly includes a drill bit (also referred to herein as a “milling device”) and an outer milling device (also referred to herein as “shoe”) that encloses the drill bit. The drill bit is often rotated by a mud motor driven by a drilling fluid supplied under pressure from the surface to the drill string. The outer milling device is typically attached to the bottom of a tubular that encloses the mud motor. In operation, the drill string is rotated clockwise such that it rotates both the drill bit and the outer milling device in clockwise direction. The mud motor also rotates the drill bit in the clockwise direction, which rotation is superimposed on the rotation of the drill string.
The disclosure herein provides a drilling system wherein the drill bit and the outer milling device rotate at different speed and in one configuration in opposite directions to improve cutting of debris and plugs formed by the drilling process.